The present invention relates to liquid circulation systems in which a liquid is circulated, usually at low pressure or negative pressure through a work station such as a mold in order to accomplish a function, such as the heating and/or cooling of the work station. In such systems it is desirable to control the pressure of the liquid in order to prevent failure of any weakened element in the system, such as rupture of hoses or seals and leakage of the liquid from the system, such as from small cracks which may be present in the work station or mold.
Liquid circulation systems, such as for mold heating, most commonly are provided with a supply line through which fresh liquid is introduced to the system, such as cool or cold water under pressure from a city supply conduit in order to supply liquid to the system and, when required, to lower the temperature of the circulating liquid, and with a drain line through which used or hot liquid can be emptied from the system. In prior-known systems it is known to employ a pressure regulator to control the pressure of the liquid entering the circulation system which includes the main circulation pump, and a supply bypass in advance of the regulator through which some of the supply liquid can be circulated by a second pump to a vent tank or reservoir. A third pump is required to pump the liquid from the reservoir back into the circulation system or out of a drain line which is normally closed but is opened when the system is to be cooled, in order to permit cold supply liquid to enter the circulation system. In such systems the circulation conduit is open at all times to the supply line and, therefore, the pressure of the supply liquid becomes the lowest pressure in the system, which pressure is increased even further by the action of the pump. When the heating cycle is completed, the drain line is opened to dump or circulate the hot liquid out of the drain and to permit cold supply liquid to enter from the supply line. This causes substantial variations in the pressure of the liquid within the system which necessitates the use of pressure check valves and safety valves in the system. These pressure variations are due to the facts that the supply liquid of city water is at an elevated pressure, i.e., about 50 psi to 80 psi, and the system is open to said pressure during circulation. Then the pressure within the circulation system drops down to the drain pressure, which may be 0 psi, during the draining cycle. Thus the system may undergo a pressure variation between about 0 psi and 80 psi, or more, which can cause failure of any weak element in the system, such as rupture of rubber hoses or seals and/or damage or leakage of the work station or mold.
In another known system which employs a single pump, the circulation water is pumped to a supply tank or reservoir located on a plane below the level of the mold and is sucked back up to and through the mold at sub-atmospheric pressure. The water pressure through the mold cannot be increased and, therefore, the rate of liquid circulation cannot be controlled at increased rates.